Nitrogen and carbon fractionation in planetary magma oceans and origin of the superchondritic C/N ratio in the bulk silicate Earth

Abstract

Volatiles are important for planetary geodynamics, climate, and habitability. The C/N ratio of the bulk silicate Earth (BSE) is superchondritic, which provides a useful tool for understanding the origin of Earth's volatiles. The Earth accreted largely from differentiated planetesimals and embryos, and the fate of nitrogen and carbon in magma oceans (MOs) of such rocky bodies is key in shaping the BSE's C/N ratio. Here we present experiments performed at 0.3–3 GPa and 1400–1600°C to determine both the solubility and partitioning of nitrogen and carbon between Fe-rich metallic and silicate melts at graphite-saturation and the saturation of N2-rich gas. The quenched samples were analyzed by electron microprobe, secondary ion mass spectrometry, and Raman spectroscopy for their N–C–H–O contents and speciation. Our results show that the C/N solubility ratios of the silicate melts (ppm/ppm by wt.) are a multi-function of pressure, temperature, silicate melt composition, and mainly oxygen fugacity (fO2), and increase from 0.01 to 1.6 with increasing fO2 from IW-3.7 to IW+0.4 (IW refers to the iron–wüstite buffer). Raman spectra and theoretical considerations reveal that the main species in silicate melts are N2, N3−, and N–H in the case of nitrogen, and CO, CO2−3, and C–H in the case of carbon. Nitrogen and carbon may also form complex species, which, however, could not be identified presently. The metal/silicate partition coefficients of nitrogen and carbon DN,Cmetal/silicate are 1–114 and 34–3050, respectively. The DCmetal/silicate/DNmetal/silicate ratios are 1.5–1100, which decrease with increasing pressure, fO2, and the water content in silicate melts. Our results imply that N–C fractionation could occur during core-formation and silicate MO degassing. For a rocky body starting with a chondritic C/N ratio, core-formation would result in a superchondritic C/N ratio in its core if that rocky body is S- and Si-poor. However, a superchondritic C/N ratio can also be achieved in the silicate mantle through C-saturation coupled with preferential nitrogen degassing and loss into space, if the rocky body is oxidized and has a S-rich core, or is reduced and has a Si-rich core. Both Earth's accretion of planetesimals and embryos with cores as the major nitrogen and carbon reservoirs, and Earth's disequilibrium accretion of C-saturated embryos through core–core merging, could have helped establish the BSE's superchondritic C/N ratio. During Earth's accretion of the last few giant impactors, multiple episodes of MO degassing and erosion-induced atmospheric loss would have also favored the formation of a superchondritic C/N ratio in the BSE, due to the oxidized nature of Earth's surface MO (fO2 > IW) and the preferential loss of nitrogen into space. Finally, we emphasize that oxidization of emulsified planetesimal cores in Earth's upper mantle during its final accretion stages could have further helped establish the BSE's superchondritic C/N ratio. Accordingly, the BSE's superchondritic C/N ratio may be an outcome of combined processes operating both on the accreted planetesimals and embryos and on the proto-Earth itself.